1. Field of the Invention
The invention relates generally to a wireless apparatus with an integral antenna device and more particularly to a GPS instrument in which the combination of an encased ground plane and wire filament functions as an electrically short linear GPS antenna.
2. Description of Related Art
GPS antennas have historically been fabricated as circular polarized antennas using either quadrifilar helices or circular patches. In order to operate efficiently, these antennas must be properly oriented towards the sky. Circular polarized antennas degenerate into linear polarization near their horizon, accordingly, replacing these antennas with a linear antenna has little effect on the received signal strength of the satellites that would be in the linear operation region of the circular polarized antenna. The strength of the peak signals received will be less because the maximum gain of the linear antenna is 3 dB less than the maximum gain of a circularly polarized antenna. This loss of signal strength is a reasonable tradeoff given the low cost and simplicity of a linear antenna.
Many modern applications for GPS do not allow for the proper orientation of a circularly polarized antenna, and circular antenna performance below or behind the main lobe of the antenna pattern can be worse than that of a linear antenna. For example, a cellular phone with a GPS receiver may be positioned such that the telephone keypad is facing up or down, furthermore, the telephone may be carried in a pocket with the keypad in a vertical orientation. Positioning the telephone as such places the circularly polarized antenna facing up, down or toward the horizon. Thus the operational efficiency of a GPS receiver that receives signals through the circular polarized antenna of the cellular telephone is generally degraded due to the inappropriate physical orientation of the antenna.
A number of wireless communication devices with integral linear antennas currently exist. For example, cellular telephones employ an extendible antenna that uses shielded circuitry as a part of the antenna, along with a wire filament that can be straight, or electrically lengthened by inductively loading one end with a coiled portion of the antenna filament. Typical embodiments of these types of cellular telephones are presented in U.S. Pat. No. 4,868,576. The antennas used in the communication device assemblies presented in the prior art are usually made as large as possible to achieve broad bandwidth. Such large antennas are neither desirable nor practical for GPS devices, which in many applications are small sized.
Hence, those skilled in the art have recognized a need for a wireless apparatus having an integral GPS antenna that is physically small, inexpensive, and functional in arbitrary orientation. The present invention fulfils these needs and others.
Briefly and in general terms, the invention is directed to a wireless apparatus having an integral antenna for receiving GPS signals. The apparatus includes an electrically conductive casing housing a ground plane and GPS receiver circuitry. The casing is electrically connected to the ground plane to form a first antenna element. The apparatus further includes a second antenna element located external to the casing. The second antenna element is electrically coupled to the first antenna element and the GPS receiver circuitry. The first antenna element and second antenna element are configured and disposed relative to each other to form an antenna for receiving GPS signals.
In a detailed aspect, the apparatus further includes a printed circuit board at least partially housed within the casing. The ground plane and the GPS receiver circuitry are carried by the printed circuit board. In another detailed facet, a portion of the GPS receiver circuitry is electrically connected to the ground plane. In yet another facet, the ground plane is embedded within the printed circuit board and the casing is electrically connected to the ground plane through the printed circuit board. In another detailed aspect, the casing substantially confines RF leakage signals from the GPS receiver circuitry to the space within the casing.
In another detailed facet, the second antenna element is directly connected to the GPS receiver circuitry through a signal port. In yet another detailed aspect, the second antenna element is electrically coupled to the first antenna element and the GPS circuitry through an inductive element electrically connected to the casing at a first connection point and to the second antenna element at a second connection point. The second connection point is further connected to the GPS receiver circuitry through a signal port.
In still further detailed facets, the second antenna element comprises a straight conductive wire filament disposed relative the first antenna element such that the first antenna element and the second antenna element function as a dipole antenna. Alternatively, the second antenna element may comprise a wire filament formed in one of a meandering, spiral, L and U shape. In another detailed aspect, the second antenna element comprises a conductive element formed on the printed circuit board. In yet another detailed aspect, the conductive element is formed on a portion of the printed circuit board that extends beyond the casing.
These and other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings, which illustrate by way of example, the features of the invention.